Understanding JVM

Understanding JVMAparna Chaudhary

Friday, October 4, 13

What’s inside my VM?

VM Memory

VM Memory



VM Memory

Guest OS Memory

VM Memory

Guest OS Memory



VM Memory

Guest OS Memory

JVM Memory

VM Memory

Guest OS Memory

JVM Memory



VM Memory

Guest OS Memory

JVM Memory

PermGen-XX:MaxPerm

Size

VM Memory

Guest OS Memory

JVM Memory

PermGen-XX:MaxPerm

Size



VM Memory

Guest OS Memory

JVM Memory

PermGen-XX:MaxPermSize

Stack-XSS Per ThreadVM Memory

Guest OS Memory

JVM Memory


Stack-XSS Per Thread



VM Memory

Guest OS Memory

JVM Memory



HEAP

VM Memory

Guest OS Memory

JVM Memory



HEAP



VM Memory

Guest OS Memory

JVM Memory



HEAP

Initial-Xms

VM Memory

Guest OS Memory

JVM Memory



HEAP

Initial-Xms



VM Memory

Guest OS Memory

JVM Memory



HEAP

Initial-Xms

-Xmx

VM Memory

Guest OS Memory

JVM Memory



HEAP

Initial-Xms

-Xmx


Exception in thread "main": java.lang.OutOfMemoryError: Java heap space

Exception in thread "main": java.lang.OutOfMemoryError: PermGen space

Exception in thread "main": java.lang.OutOfMemoryError: GC overhead limit exceeded

Exception in thread "main": java.lang.OutOfMemoryError: Unable to create new native thread

and more...

OutOfMemoryError


Exception in thread "main": java.lang.OutOfMemoryError: PermGen Space

OutOfMemoryError


Perm Gen Space

Used for Storing the JVM’s internal representation of Java Classes


Life of a class

described by

loaded by

references

references

Object

Class

Classloader

described by

loaded by

references

references

Object

Class

Classloader


Life of a class

described by

loaded by

references

references

Object

Class

Classloader

references

described by

loaded by

references

references

Object

Class

Classloader

references


Life of a class

Object

Class

Classloader1

references

referencesObject

Class

Classloader2

references

references

Object

Class

Classloader1

references

referencesObject

Class

Classloader2

references

references


Life of a class

Object

Class

Classloader1

references

referencesObject

Class

Classloader2

references

references

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Object

Class

Classloader1

references

referencesObject

Class

Classloader2

references

references

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class

Class


OOM - PermGen

This occurs when JVM wants to load new class definitions; but there is not enough space in PermGen space


OOM - PermGen

SymptomAt server startup

CauseBigger PermGen footprint

ResolutionIncrease PermGen capacity -XX:MaxPermSize


OOM - PermGen

SymptomPermGen space increases gradually

CausePossible classloader leak

ResolutionInvestigate any third party API you are using for any potential class loader leak defect


OOM - PermGen

SymptomOOM after deploy-redeploy cycles

CausePossible classloader leak in the App Server

ResolutionSomeone should fix it for you. Hmm..where is EAP?


OutOfMemoryError

Exception in thread "main": java.lang.OutOfMemoryError: GC overhead limit exceeded


OOM - GC overhead limit exceeded

Policy to allow the VM to detect potential OutOfMemoryError conditions earlier and before it runs out of Java Heap space.

Can be turned off -XX:-UseGCOverheadLimit

But don’t!


OOM - GC overhead limit exceeded

When is this error triggered?

✓Too many Full GC iterations✓Too much time spent in GC


Exception in thread "main": java.lang.OutOfMemoryError: Unable to create new native thread

OutOfMemoryError


OOM - Unable to create new native thread

JVM is asking a new thread from the OS and the underlying OS cannot allocate a new thread anymore.


Why?

VM Memory

Guest OS Memory

JVM Memory



HEAP

Initial-Xms

-Xmx

not enough stack space


OOM - Unable to create new native thread

✓Check ulimit - process limit✓Consider using thread-pools✓Reduce heap space or perform vertical scaling


Exception in thread "main": java.lang.OutOfMemoryError: Java heap space

OutOfMemoryError


JVM Heap Structure

Eden S0 S1 Tenured

Old Generation

YoungGeneration

SurvivorSpace


Eden

To From

Survivor Spaces

Object Allocation

Young Gen = Eden + Survivor SpacesFriday, October 4, 13

Eden

To From

Survivor Spaces

Just allocatedare allocated in Eden

Object Allocation

Young Gen = Eden + Survivor SpacesFriday, October 4, 13

Eden

To From

Survivor Spaces

Filling up Eden Space


Eden

To From

Survivor Spaces

Just allocatedare allocated in Eden

Filling up Eden Space


Minor GC - Copy referenced objects

Eden

To From

Survivor Spaces Unreferenced

Referenced


Minor GC - Copy referenced objects

Eden

To From


Referenced

1 1 1 1


Next Minor GC - Object Aging

Eden

From To


Referenced

1 1 1 1



Eden

From To


Referenced

1 1



Eden

From To


Referenced

1 2



Eden

From To


Referenced

2 2



Eden

From To


Referenced

2 2 1



Eden

From To


Referenced

2 2 1 1


Next Minor GC - Additional Aging

Eden

To From


Referenced

2 2 1 1



Eden

To From


Referenced

2 1



Eden

To From


Referenced

321


Next Minor GC - Promotion

Eden

From To

Survivor Spaces

Unreferenced

Referenced

3211 14

Tenured


Next Minor GC - Promotion

Eden

From To

Survivor Spaces

Unreferenced

Referenced

3211

14Tenured


Major GC

Eden

From To

Survivor Spaces

Unreferenced

Referenced

3211 1

Tenured


Warning!

This knowledge does not make us JVM experts!

Always focus first on software before hacking JVM.


JVM Diagnosis

jmap

jhat


Class Histogram

jmap -F -histo `pgrep java`

Make it a practice to proactively use this command after a load test and endurance test.


Heap Dump

jmap [ option ] <pid>

jmap -F -dump:live,format=b,file=/var/opt/jboss-as/tmp/io/heapdump.hprof `pgrep java`http://docs.oracle.com/javase/6/docs/technotes/tools/share/jmap.html


What Next?

Demo

VisualVM

Eclipse Memory Analyzer Tool

Other tools and commands

GC Algorithms (Optional)


Understanding JVM

Technology

Transcript of Understanding JVM